Processes for producing lower aliphatic monocarboxylic acids such as acetic acid by the vapor phase oxidation of lower aliphatic hydrocarbons such as butane are known. For example, acetic acid is prepared by the vapor phase oxidation of butane according to the following equation: C.sub.4 H.sub.10 + 5/2 O.sub.2 .fwdarw.2CH.sub.3 COOH + H.sub.2 O. Such processes frequently involve the use of catalysts, as shown for example in Russian patent No. 166,670, which discloses such a process utilizing a vanadium pentoxide catalyst.
The use of vanadium pentoxide catalysts, either supported or unsupported, for the vapor phase oxidation of lower aliphatic hydrocarbons results in yields and process efficiency which fall substantially short of theoretical potential. Also, the resulting products are often impure due to a lack of selectivity when such processes are employed.
Neat (i.e. unsupported) vanadium tetroxide has been suggested as a remedy for the above disadvantages but the use of this catalyst in the vapor phase oxidation of lower aliphatic hydrocarbons also results in inefficient processes which lack a high degree of selectivity. Further, neat vanadium tetroxide lacks the physical strength of a supported catalyst and also lacks sufficient heat transfer characteristics for this highly exothermic reaction. Accordingly, hot spots are produced in the catalyst bed during a vapor phase oxidation.
In addition, when using neat vanadium tetroxide, it is necessary to maintain a careful balance between temperature and the ratio of lower aliphatic hydrocarbon to oxygen in order to prevent the neat catalyst from being oxidized to vanadium pentoxide.
Another problem which occurs when neat vanadium tetroxide is used as a catalyst for the vapor phase oxidation of lower aliphatic hydrocarbons is that substantially all (i.e., more than 75 percent) of the total conversion occurs in the first portion of the catalyst zone or bed (i.e., the first 25 percent of the total catalyst zone or bed) which is in contact with the reactants. This concentration of conversion in the first portion of the zone in an exothermic reaction raises the exotherm temperature at that point substantially in excess of that at later points in the catalyst zone. These high exotherm temperatures make control of the reaction more difficult, dictate more expensive heat-resistant materials, often decrease the yield of desired product and/or increase the yield of undesired by-products and are otherwise disadvantageous.